This happened to the PO of my '86 300E. The shop replaced the fuel and OVP relays. On my Volvos, the Bosch relays tend to develop tiny cracks in the soldering on the PC board. Eventually the cracks open the circuit and the engine stalls. When the current stops flowing through the solder, the solder cools and the crack closes. The engine will then start and run until the solder heats up again.

You can replace the relays, $$$$, or open them up and resolder the PC boards, cheap fix. Bosch make relays for both Volvo and MBZ and they have this same problem.